To achieve optimal production from a tobacco crop, growers commonly decapitate ("top") their plants in order to obtain leaves with desired physical properties and chemical composition. The topping process involves removing the stem apex, including flowers and some top leaves, near the stage of plant maturity to stimulate the development of the remaining leaves. Current practice frequently involves topping tobacco plants in the "button" stage, soon after the floral part of the plant begins to appear. As a result of the topping process, tobacco plants produce axillary buds, or "suckers," which must be controlled in order to produce the most desirable tobacco plant. These suckers have to be continuously removed to achieve the purpose of topping. When tobacco plants are not topped for about three weeks after reaching the button stage, yields can be reduced by about 20-25 lbs/acre--about 1%.
In the past, the removal of suckers has been carried out by hand, which is a laborious and time-consuming process. Certain chemical growth-agents have been suggested and are now used by many tobacco growers to achieve economical control of suckers. While some of these chemicals provide adequate inhibition of sucker development and are in commercial use, some chemicals cause a variety of metabolic changes in the tobacco plant and result in leaves of undesirable quality. In addition, there has been some question of undesirable residues of chemical agents in the leaf tissue.
One chemical compound which is extensively used for sucker control in tobacco plants is maleic hydrazide. The use of this compound is particularly common with flue-cured tobacco. Flue-curing is a procedure which is widely used by tobacco growers in the United States wherein heat is applied to the tobacco leaves as a part of the curing process. Fatty alcohols are also extensively used in sucker control programs. As described below, contact agents such as the fatty alcohols are used in an initial stage of sucker control, followed by treatment with a systemic agent such as maleic hydrazide.
Typical procedures for reducing or preventing sucker growth in topped tobacco plants include about 2 to 3 initial treatments with contact agents followed by application of a systemic compound such as maleic hydrazide. As this name implies, contact agents reduce sucker growth by inhibiting the suckers with which these agents come into contact. This direct action on the vegetation is in contrast to systemic products, such as maleic hydrazide, which work by altering the physiology of the plant in such a way so as to inhibit axillary growth. In current practice, fatty alcohols are commonly used as contact agents. Contact alcohol chemicals desiccate tender sucker tissue, while the systemic chemicals retard sucker growth by inhibiting cell division. The use of a contact alcohol allows for earlier topping, which increases yields. Therefore, its purpose is to provide sucker control during the period between early topping and the time the upper leaves are large enough to be sprayed with a systemic chemical without causing injury.
One product which is now commonly used in sucker control programs is flumetralin (Prime+.TM.). This product works through a mechanism involving cellular inhibition but must actually contact the sucker tissue to be effective. Therefore, flumetralin is not truly a systemic agent. Flumetralin can be applied individually or in a tankmix with maleic hydrazide.
Recently, there has been increased concern over residue levels of maleic hydrazide which remain in tobacco after the crop is harvested. Despite this concern, residue levels of maleic hydrazide for flue-cured tobacco showed no significant improvement from 1990 to 1991. One study found that the average maleic hydrazide residue in 1990 was 147 ppm, and in 1991 the average was 140 ppm. European countries are particularly sensitive to the maleic hydrazide problem. In Germany, for example, the recommended standard level is 80 ppm maleic hydrazide in the finished product, while in Italy and Spain, the maximum permissible level is 80 ppm. When the maleic hydrazide in the raw tobacco exceeds the level permitted in the finished product, the cigarette manufacturer must blend the high-maleic hydrazide tobacco with other tobacco with less or no maleic hydrazide. The obvious result is a decreased demand for tobacco with high maleic hydrazide residues. It is expected that the European unification process will ultimately result in 80 ppm maximum applying to all European countries. Since almost 50% of the flue-cured tobacco exported to the United States goes to Europe, the potential adverse economic impact of excessive maleic hydrazide residues is quite significant.
A variety of compounds have been used over the years in attempts to inhibit axillary bud growth. For example, esters of fatty acids have been reported to inhibit axillary bud growth in tobacco (Tso, T. C. [1965]J. Agr. Food Chem. 13(1):78-81; Tso, T. C., Canadian Patent No. 968175 issued May 27, 1987; and Tso, T. C., Canadian Patent No. 968176 issued May 27, 1975). Methyl esters of fatty acids have also been reported to be useful as chemical pinching agents (Sill, L. Z., P. V. Nelson [1970]J. Amer. Hort. Sci. 995(3):270-273; Nelson, P. V., R. K. Reid [1971]Amer. J. Bot. 58(3):249-254). Fatty acids themselves, however, have long been regarded as being too phytotoxic to be useful as chemical pinching agents or axillary bud inhibitors (Tso, T. C. [1964] Nature 202(4931):511-512).
There is a great need for new methods of tobacco sucker control which achieve the desired level of control but reduce the amount of potentially dangerous chemical residues left on the tobacco leaves.